(1) Field of the Invention
This invention relates to a synthetic polyester pulp and a process for preparing the synthetic polyester pulp. The polyester pulp has a very fine and relatively uniform fibril structure and can be made advantageously into sheet-like products on conventional paper-making machinery.
(2) Description of the Prior Art
Heretofore, synthetic pulps made of a synthetic, highly polymeric material and synthetic papers made from the pulp by a wet sheet-forming process have been proposed. The conventional synthetic pulps are prepared by various processes which are classified into the following four processes. In a first process, a solution of a highly polymeric material is coagulated by pouring the solution into a poor solvent, which is incapable of dissolving or capable of slightly dissolving the highly polymeric material, while a vigorous shearing and/or beating action is applied to the poor solvent (refer to British Pat. No. 868,651). In a second process, a mixture of two or more synthetic polymeric materials, which are incompatible with each other, is melt-spun into filaments. After the filaments are cut into short lengths, the filaments are mechanically split into fibrils (refer to Japanese Patent Publication No. 9651/1960). In a third process, a mixture of two or more synthetic polymeric materials, which are incompatible with each other, is melt-spun into filaments. After the filaments are cut into short lengths, the filaments are immersed in a solvent capable of selectively leaching one of the polymeric materials (refer to U.S. Pat. No. 3,382,305 ). In a fourth process, a solution of a synthetic polymeric material maintained at a temperature higher than the boiling point of the solvent used and at a high pressure is ejected into an environment of a low pressure, and the ejected product is beaten (refer to Japanese Patent Publication No. 16,460/1961).
However, it is difficult to prepare synthetic pulps from polyesters, particularly polyetheylene terephthalate, having good mechanical, chemical and electrical properties, by the above-mentioned processes. Namely, even when polyesters are intended to be processed by the above-mentioned first process, there is no solvent suitable for dissolving the polyesters. When polyesters are processed by the above-mentioned second process, the mixed polymer filaments cannot readily be mechanically split into fibrils because polyesters have high mechanical strengths. When polyesters are processed by the above-mentioned third process, first, there is no satisfactory solvent used for selectively leaching the polymers (such as polyamides) incompatible with the polyesters, and, secondly, the resulting pulp is comprised of fibrils which are neither arborescent nor fluffy, and therefore, the resulting paper has poor mechanical strengths. When polyesters are processed by the above-mentioned fourth process, first, there is no satisfactory solvent and secondly, this flash spinning process is costly.
The synthetic polyester pulps prepared by the above-mentioned processes have some disadvantages. For example, the polyester pulp prepared by the first process is characterized as possessing a fibrid structure comprised of fibrous, ribbon-like or foliate stems having tentacular projections. This fibrid structure has relatively large dimensions and, the substantial part of the fibrid structure has a dimension of at least about 10 microns, although only a minor part thereof has a dimension of about one micron. The polyester pulp prepared by the third process is of an oriented fibril structure comprised of fine fibrils entangled with each other. The fibrils of this pulp have relatively round and regular cross-sectional shapes, and the pulp is not completely satisfactory in adaptability to paper making machinery.